a. Field of the Invention.
The invention relates generally to the processing of analog or digital signals and in particular to a circuit for use in amplitude spectrum equalization and differentiation of signals the peaks of which are representative of digital data.
b. Prior Art.
In signal processing and in particular the magnetic data storage and retrieval art, equalization or waveform compensation is the term applied to the process of shaping signals, e.g. those picked up by a magnetic read head in digital magnetic recording, so that data bits can be recovered accurately. The readback signal of digital magnetic recording is a summation of isolated pulses each of which is similar to a bell-shaped curve and can be characterized as a Lorentzian pulse in the time domain. The peak of each of these isolated readback pulses represents a written transition on the storage medium. These readback pulses are essentially unbound in time and have long tails preceding and following the peak of the pulse. Therefore one reason that equalization is necessary in magnetic storage playback is that magnetic flux transitions are recorded so densely on the storage medium, that adjacent readback pulses can influence each other to cause pulse crowding or pulse interference. This pulse crowding phenomenon results in peak shift or bit shift and also pattern dependent amplitude variation. By means of amplitude spectrum equalization of the unequalized readback isolated pulses, the after equalization pulse is completely bound in time and its signal level returns to zero in a relatively shorter time and remains there. If the signal level of each equalized isolated pulse drops to a nearly zero signal level before the occurrence of the peak of the next pulse, then pulse interference or pulse crowding is eliminated. This pulse slimming or pulse narrowing technique corrects for peak shift and variable peak amplitude problems in the unequalized readback signal waveform. This correction means yields a readback signal waveform whose amplitude is frequency and data independent, i.e. a constant peak amplitude output signal, and whose peaks have negligible peak shift. An amplitude equalizer should cause minimum degradation of the signal to noise ratio and also cause a minimum amount of ringing on the baseline of the equalized isolated pulse.
In equalizers of the prior art, such as in U.S. Pat. No. 4,081,756 to R. Price, G. Jacoby and A. Geffon the equalized signal is transmitted to a peak detection channel which includes a differentiation circuit to define the peak positions of the signal and a detector, such as a zero crossing detector, produces digital pulses representing peaks of the readback signal, which consist of true data peaks and noise induced peaks. By means of the equalization and ideal differentiation, the timing accuracy of the relative peak positions of the readback signal representing the digital information written on the medium is maintained. In the aforementioned patent the equalized signal is also transmitted to another amplitude detection channel, referred to as the "gate generator", to produce pulses or gates whenever the constant peak amplitude signal exceeds a predetermined threshold. These gates, corresponding to the true data peaks, are used to examine and indicate the true data peak positions detected by the peak detection channel. In short, use of an equalizer to shape the amplitude spectrum of the input signal so as to slim the isolated pulse and compensate the amplitude of the composite waveform can increase the magnetic storage density.
The aforementioned patent shows a balanced equalizer in FIG. 11. Two separate delay lines were terminated by emitter followers to approximately simulate open circuit termination in an attempt to provide total signal reflection. An input to each delay line was cross-connected to the output of the other delay line through another emitter follower and the two cross-connects were used as inputs to a differential amplifier, followed by a Bessel filter. This equalizer is suitable for shaping the input signal to a dual channel signal detector, as described in the patent. One channel, including a differentiator, is used for peak detection. The other channel is used for amplitude detection, i.e. gate generation, over a threshold level. The outputs of both channels are fed to an AND gate which produces the digital output readback signal. Note that equalization and differentiation in the above patent were achieved using different circuits.
In U.S. Pat. No. 3,516,066 issued on June 2, 1970, G. V. Jacoby discloses an equalizer comprising amplitude and phase compensation circuits followed by a delay line differentiator. The theory of delay line differentiators is also discussed in an article entitled "The Use of Delay Lines in Reading a Manchester Code" by T. H. Chen in IEEE Transactions on Computers, September, 1968.
In the book entitled "Data Transmission", by Bennett and Davey, p. 269, there is a discussion of transversal filters formed by tapped delay lines. The author points out that any correction in amplitude characteristics of a signal with respect to frequency can be brought about by a transversal filter. Note that such a transversal filter has a single ended input and single ended output, and is therefore unbalanced.
In U.S. Pat. No. 3,408,640 issued on Oct. 29, 1968, Masson discloses two separate tapped delay lines for use in reading high density magnetically recorded data. Selected taps in each branch are connected to a resistor summing network for equalizing data waveforms. The two separate delay line branches are terminated in their characteristic impedances and are used in an unbalanced manner like the transversal filter.
While the equalizers of the prior art are quite useful, there is a need for improved equalization. For optimum performance in read signal detection, it is desirable to have two separate amplitude spectrum equalizers, one for the peak detector and one for the gate generator, and it is also desirable to have a delay line differentiator. However, implementation of separate delay line equalizers and a differentiator into the system is not only expensive but also generates more electronic noise and delay line matching difficulties.